I do food safety work and have done a lot of research on the field-to-fork timeline. A lot of produce IS washed before it gets to you, but a lot is not very thoroughly washed. I use a basic vinegar/water in a spray bottle and scrub them (unless they are berries, then I spray and rinse them). Here are the reasons:
1. the FDA says that all fruits and vegetables, including those that are organically grown, should be thoroughly washed to remove soil, surface microbes, and some pesticides (
http://www.fda.gov/food/resourcesforyou/consumers/ucm114299).
2. Tons of people are handling your food. From the person who picks it, packs it, to the person who unpacks it and puts it on the shelf. No one is wearing gloves and I just prefer to wash that off.
3. Luke LaBorde, associate professor in Penn State University's Department of Food Science says to scrub your food if it has a waxy or thick textured skin. Potatos and melon grooves mean your hands can't rub off all the dirt. Waxy-skinned citrus fruit and cucumbers may have pathogens sticking to the exterior.
Unlike what the PP said, E.Coli is often simply on the surface and can be washed off.
Laborde notes, "There have been several microbial outbreaks involving cantaloupes because they're grown near the ground and can pick up dirt." The dirt may contain microorganisms spread through poor irrigation techniques in the field. These pathogens tend to thrive on the grooved surface of a cantaloupe.
LaBorde stated: "When you cut open the cantaloupe," he says, "you can transfer bacteria to the fleshy part inside." And because a cantaloupe is not an acidic fruit — unlike, say, a tart apple — bacteria can grow more easily on the fleshy part.
Basically, it's not worth the risk to me for something that takes just a few seconds. I think plain water is absolutely fine. I like the vinegar wash because a This American Kitchen (Cooks Illustrated) did a test to see whether water alone or cut vinegar was better for removing bacteria from the surface of produce and the vinegar won.
Either way, you should certainly wash your produce.